This application claims the priority of Korean Patent Application No. 2003-91340, filed on Dec. 15, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a semiconductor light source which generates ultrafast optical signals using beats of a dual-wavelength laser, and more particularly, to a tunable light source capable of continuously adjusting an interval between two wavelengths, and a method of manufacturing the same.
2. Description of the Related Art
Microwaves or millimeter waves through optical fiber have been researched since they can be applied to wireless access networks and phase array antennas. However, electrical approaches in the range of microwaves or millimeter waves are limited in a transmission distance and incur high expenses. Unlikely, light in a THz range transmits non-linear materials and thus can be used to produce solitons or process images. For this reason, methods of optically generating ultrafast signals having ultrahigh frequency in a range of GHz through THz have been researched and developed.
In a representative method, ultrafast optical signals can be generated by heterodyning two laser diodes whose lasing wavelengths are different by a desired frequency. Where a beat frequency of signals generated by the two laser diodes is converted into an electrical signals using a photodetector, a desired electrical beat signal having an ultrahigh frequency can be obtained. Such an optical mixing method is advantageous in generating microwaves or millimeter waves having a great power. When using optical fiber based communication systems, complexity of base station equipment is avoidable in a field of mobile communication.
S. llo proposed a structure enabling dual-mode pulsation by periodically shifting the phase of a grating when manufacturing a structure in which an active area and a Distributed Bragg Reflector (DBR) are integrated in a monolithic way [IEEE Photonics Technology Letters, vol. 7, pp. 959–961, 1995]. However, the proposed structure does not allow the mode interval between two lasing wavelengths to be changed, requires conditions for reliable lasing operation, complicates manufacturing processes since e-beam lithography is required to periodically shift the phase of a grating.
To overcome these problems, a device for generating ultrahigh-frequency optical signals using a sampled grating DBR employing a holographic method is disclosed in IEEE Electronic Letters [vol. 35, pp. 472–474, 1999]. However, the device requires very critical conditions to maintain the intensities of two lasing optical signals the same. Moreover, it is impossible to continuously change a wavelength interval (i.e., detuning) between two lasing signals,.
A device disclosed in IEEE Transaction on Photonics Letters [vol. 11, pp. 15–17, 1999] can continuously adjust an interval between lasing wavelengths, but intensity of light having two lasing wavelengths is not uniform. Moreover, e-beam lithography is required to manufacture gratings having different periods.